A switched reluctance motor (SRM) is energized phase by phase in sequence to generate reluctance torque and enable smooth motor rotation. A schematic diagram of a three phase switched reluctance motor is shown in FIG. 1. The number of strokes (N) in SRM per one mechanical revolution is dependent on the number of phases (M) and the number of rotor poles (P) and is given by,N=M*P  (1).
Therefore, the stroke angle(S) in mechanical degrees is defined as,S=360°/N  (2).
When the number of poles is very large and the stroke angle is very small, the SRM is typically operated in open loop as a variable reluctance stepper motor and needs no knowledge of rotor position information during running condition. On the other hand, when the number of poles is small and the stroke angle is very large, the SRM is generally operated in closed loop during running condition and hence, the knowledge of accurate rotor position information is very important to rotate the motor.
Accurate rotor position information is typically obtained from a shaft position sensor. Shaft position sensors are expensive and have reliability problems and hence, the sensorless operation of SRM is desired.
In a typical position sensorless operation of SRM, rotor position estimation is carried out either discretely i.e. once per stroke angle or continuously. Discrete rotor position estimation is ideal for applications where slow speed response is required where as, continuous rotor position estimation is carried out in applications where fast speed response is desired. Rotor position estimation of the SRM can be carried out from the pre-determined knowledge of its non-linear per phase flux-linkage/current characteristics or the inductance/current characteristics as shown in FIG. 2 and FIG. 3 respectively. Several non-linear analytical models based on the flux-linkage and the inductance characteristics of the motor with respect to the rotor position have been proposed [S. Saha, K. Ochiai, T. Kosaka, N. Matsui and Y. Takeda “Developing a sensorless approach for switched reluctance motors from a new analytical model”, 1999 IEEE-IAS Annual meeting, Vol. 1, pp 525-532., and G. Suresh, B. Fahimi, K. M. Rahaman and M. Eshani “Inductance based position encoding for sensorless SRM drives”, 1999 IEEE-PESC Annual Meeting, pp 832-837.] to calculate the exact rotor position after sensing the d.c. link voltage Vdc and the phase current Iph of the inverter drive circuit as shown in FIG. 4. A voltage sensor VS is used to sense the d.c. link voltage and three current sensors CS are used to sense the current flowing in each phase winding 1u, 1v or 1w of the SRM 1. The traditional lock and forced drive method with open loop operation can be applied for the starting of these motors at no load. Rotor position estimation scheme is enabled at a minimum speed (ωmin) which is normally 10% of the rated speed or higher. The motor moves from open loop to closed loop operation after the rotor position estimation scheme is enabled and then subsequently load is applied to the motor. The entire closed loop control scheme with the rotor position estimation is implemented with the help of a micro-controller or a digital-signal processor.